While Nick Clegg's performance in the first leadership debate has suddenly made the Lab-Con stranglehold on power look weaker, one of the most dramatic political developments of a generation has gone relatively unnoticed.

With their new manifesto, the Greens have transformed themselves from a single-issue outfit into Britain's new – and only – leftwing party. They have pledged soak-the-rich tax hikes, aimed at radically redistributing Britain's wealth and delivering social justice for all.

They will use some £112bn in new taxes by 2013 to fund higher minimum wages and pensions, free home insulation, free social care for the elderly, big tax breaks for people on lower incomes, as well as the expected huge investment in transport.

Perhaps it's understandable that this hasn't received much attention – the Greens could only win one parliamentary seat, at most, this election.

But it's short-sighted not to see the wider significance of this shift. It's true that our first-past-the post system means the Green manifesto is of almost no relevance to the outcome on 6 May. But think ahead to the local elections.

The Green party is positioning itself, politically, as a radical alternative to the near-identical policy platforms trotted out this Thursday on ITV. The Greens are now aiming their message not just at the growing numbers who care about the environment, but those concerned about social justice as well. Given the extent to which New Labour has moved to the centre, this will hold powerful sway among disenchanted one-time Labour voters, as well as with more radical left-wingers (note that Beatrix Campbell, a former Communist, is standing as a Green candidate in Hampstead and Kilburn). Meanwhile, taxes on airplanes and the other environmentally-focused policies appeal across the political spectrum: climate change, in Britain at least, is not exclusively a concern of leftist voters.

Here's where the local elections come in. As they often see voters expressing dissatisfaction with the ruling government, and as seats on the local council are won ward by ward, these elections offer much more scope for boosted Green support to show itself. And if the party makes significant gains in local councils, it will be in a strong position to lobby not only for better environmental policies, but for, say, higher council taxes, higher local education spending and so on – particularly if the Conservatives mean a jot of what they say about empowering localities. So, in the end, the wider appeal of the Green manifesto could come to matter a lot.

And here's another fanciful thought. If the Tories do fail to win a majority this election, but only lack one seat, and Green leader Caroline Lucas takes Brighton Pavilion – what then?